Cellulose pulps which contain fibers that offer improved strength to paper webs are in increasing demand. Fibers which offer improved strength give the papermaker the option of reducing weight or including fibrous or non-fibrous filler material to reduce cost and/or amplify other properties of paper such as optical or tactile qualities. Further, as the world's supply of native fiber becomes increasingly scarce and more expensive, it has become necessary to consider lower cost, more abundant sources of cellulose to make paper products. This has caused a broader interest in papermaking with traditionally lower quality sources of fiber such as high lignin-content fibers and hardwood fibers, as well as fibers from recycled paper. Unfortunately, these sources of fiber often result in the comparatively severe deterioration of the strength characteristics of paper compared to conventional virgin chemical pulp furnishes.
Because of the above-mentioned reasons, methods of increasing the strength potential of fibrous pulps are currently of great interest. One well known method of increasing the tensile strength of paper made from cellulose pulp is to mechanically refine the pulp prior to papermaking. However, while additional refining increases the tensile strength, it invariably reduces the rate at which water will drain through a mat of the cellulose fiber composition. Such impaired drainage can reduce the efficiency of high speed papermachines by retarding the bulk removal of water and subsequent drying of the traveling paper web.
Another method for increasing the paper strength potential is to add chemical strength additives (e.g. resins, latexes, binders, etc.) to the pulp furnish to augment the natural bonding which takes place between cellulose fibers during the papermaking operation. While such strength additives are comparatively successful, they can add significantly to the cost of raw materials to make the paper and are often accompanied by a reduction in the efficiency of the papermaking operation as well.
It is also taught in the art to fractionate cellulose fibers to obtain the fractions most suited to making certain types of papers. See, for example, U.S. Pat. No. 3,085,927, Pesch, issued Apr. 16, 1963, incorporated herein by reference. Pesch teaches the centrifugal separation of heterogeneous mixtures of springwood and summerwood fibers into fractions predominantly composed of each singular type of fiber. Additionally, Pesch's centrifugal separation, which distinguishes between fibers having different apparent specific gravity, can yield a springwood pulp having higher tensile strength. While such a procedure is somewhat effective at increasing the tensile strength, the tensile strength at a given level of drainage resistance is not greatly improved.
Other exemplary art includes U.S. Pat. No. 3,791,917, Bolton, issued Feb. 12, 1974. Bolton teaches that layered kraft paper with improved properties can be made by classifying fibers by length and relegating each length classification to its own layer in the structure. Methods of classifying which separate fibers by their length are effective at yielding a high strength fraction, i.e., the long fiber fraction. However, long fibers cause difficulties in papermaking because of their greater tendency to entangle, resulting in the production of flocks which detract from the appearance of the paper and degrade properties which are sensitive to uniformity.
Accordingly, it would be desirable to provide a cellulose pulp that offers a higher level of uniformity and tensile strength at a particular level of drainage resistance. It would further be desirable to achieve the strength improvements without having to add expensive chemicals to the pulp. Finally, it would be desirable to accomplish the improvement in strength without any concurrent substantial increase in the fiber length.
It is therefore an object of this invention to provide a cellulose pulp offering improved strength.
It is another object of this invention to provide a cellulose pulp offering a higher paper strength at a particular level of drainage resistance as compared to conventional cellulose pulps.
It is a further object of this invention to provide a cellulose pulp offering improved paper strength at a particular level of drainage and at a particular fiber length relative to conventional cellulose pulps.
These and other objects are obtained using the present invention, as will be seen from the following disclosure.
All percentages, ratios and proportions herein are by weight, unless otherwise specified.